Within disk drives, characterization circuits in the channel chip (i.e., PRML) are typically provided to measure head and media parametrics, thereby eliminating a need for external equipment such as an oscilloscope or a spectrum analyzer. Currently, these measurement circuits, such as the harmonic sensor circuit described in U.S. Pat. No. 5,784,296, obtain head/media measurements with all transfer functions of the calibration circuit locked in fixed, non-adaptive states. A particular application of the harmonic sensor is to measure changes in fly height of the playback head. Changes in fly height are important to disk drives (or tape drives) since these critical changes are indications of potential drive failure from such as unwanted contamination and sensitivity to atmospheric pressure, or sensitivity to mechanical parameters in the drive such as tape tension in a tape drive. However, a limitation of the harmonic sensor is that changes in the transfer function characteristics of the channel chip (such as, i.e., a PRML chip) due to environmental factors can be mistaken for changes in a head or media parameter, thereby corrupting the measurements sensed by the harmonic sensor. Specifically, environmental variations such as temperature, or supply voltage can change the gain of the transfer functions in the front end of a channel chip. These changes in circuit characteristics are not distinguishable from changes in the head/media interface in current measurements by the harmonic sensor.
An additional drawback of the harmonic sensor technology described in U.S. Pat. No. 5,784,296 is that the by holding the transfer function of the drive in a static mode (i.e. not allowing the chip to adapt to its environment), the detector's performance can be degraded by changes in the signal. Thus, for example, if the user wishes to perform a test such as measuring changes in fly height versus atmospheric pressure, the equalization of the incoming signal should be identical to that used when the test is started. Because the playback signal changes with atmospheric pressure, the equalization should correspondingly change to compensate for the new signal. However, the transfer functions are locked in non-adaptive state during head/media parametric measurements, thus resulting in misequalization and increased error rate. The increased error rate can limit the range over which the harmonic sensor can be used.
There is therefore a need to measure variations in head/media parameters, and distinguish those measurements from environmentally sensitive transfer function characteristics so to more accurately acquire measurements necessary for measuring fly height, conducive for detecting drive failure before data lost.